1. Field of the Invention
This invention relates to electrical resonators employing a mechanical transducer and more particularly to a method for fine tuning such resonators following batch fabrication.
2. Description of Related Art
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that meets these needs is constructed from mechanical resonators such as acoustic resonators. These devices use acoustic waves, bulk longitudinal waves for example, in thin film material, typically but not exclusively piezoelectric(PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The resonator may be suspended in air, supported along its rim, or may be placed on an acoustic mirror comprised of a plurality of alternating layers of high and low acoustic impedance (the product of speed and density), usually silicon dioxide and aluminum nitride. When an electric field is applied between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. For certain crystal orientations, such as having the c axis parallel to the thickness of an Aluminum Nitride film, the sound waves propagate in the same direction as the electric field and reflect off of the electrode/air or electrode/mirror interface.
At a certain frequency which is a function of the resonator thickness the forward and returning waves add constructively to produce a mechanical resonance and because of the coupling between mechanical strain and charge produced at the surface of a piezoelectric material, the device behaves as an electronic resonator; hence, such devices combined in known architectures can act as a filter. The fundamental mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the piezoelectric plus electrode layers. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be more compact than dielectric cavity resonators. Resonators for 50 Ohm matched applications in the GHz range may be constructed with physical dimensions approximately 100 micrometers in diameter and few micrometers in thickness.
The resonant frequency of the resonator is a function of the acoustic path of the resonator. The acoustic path is determined by the distances between the outer surfaces of the electrodes. When batch producing resonators on a substrate, the thickness of the transducing material and the electrodes is fixed at fabrication; hence, the resultant resonance frequency is also fixed. Since there are variations in thickness from device to device resulting from manufacturing tolerances, some method for fine tuning the resonance frequency of each device is needed.
To compensate for this inability to reliably and inexpensively mass produce resonators with the proper resonance characteristics, it is known to intentionally produce resonators having a lesser thickness than the thickness indicated to achieve a desirable resonant frequency, and then deposit excess material on at least one of the electrodes to change the overall thickness of the device and thereby fine tune the device. As this deposition of material may be done while the device is subjected to an input signal and simultaneously tested for resonance this method has produced acceptable results.
This method is not, however without problems as the presence of a mask needed to control the deposition over the desired electrodes creates problems of its own. If the mask, for instance is in contact with the electrode, the mask mass is added to the device mass and alters the resonance characteristics of the device. On the other hand if the mask is not in contact with the device the control of the deposition area suffers. Such masking techniques have been successful with quartz type resonators that are much larger, but have not been as successful with resonators of the order of less than one millimeter.
It has also been proposed to remove material from the device in order to adjust its resonant frequency by etching material off the top electrode of a resonator. With current technology, however, etching is not as controlled a process as deposition. Etching tends to be less uniform, smooth or reproducible than deposition. In fact prolonged etching may in cases change the composition, morphology, grain nature or roughness of thin films. Accurate etching processes require precise knowledge of the rate at which material is removed to permit stopping at the exact moment that sufficient material has been removed to produce the desired resonant frequency. To a certain extent lack of precise control of the etching rate may be alleviated by monitoring the device frequency during the etching process.
When removal of material is done in a dry etching process it is usually possible to monitor the resonant frequency of the device during the etching process. However, monitoring of the resonant frequency during etching is not possible when wet etching processes are used. Wet processes are desirable as they are much faster than dry processes.
There is thus still a need for a process to accurately fine tune a mechanical resonator to a desired frequency without concern for possible over-etching and without need to monitor the frequency during the etching process.
The above object is obtained in accordance with this invention by a method for adjusting the resonant frequency of a mechanical resonator, the method comprising using alternating selective etching to remove distinct adjustment layers from an electrode comprising a plurality of stacked adjustment layers, each of said adjustment layers having distinct etching properties from any adjacent adjustment layers.
Such a process alleviates the need to know the precise rate of etching in a particular process because etching will stop when the etching process removes all the material of one layer and reaches the next layer that is selected to be impervious to the etching process. In other words the etching stops each time at the barrier, i.e. the change from one material to another, as each layer is sequentially removed.
Because the stacked layers have been created by deposition of material on the top electrode, complete removal of each layer maintains the uniformity of the remaining layer obtained during the deposition of this layer. The composition and morphology of the unetched layer film remains ideal.
In more detail, the proposed method is a method of manufacturing a mechanical resonator having a desired resonant frequency by a process comprising:
(a) forming a first electrode;
(b) forming a transducer layer over the first electrode;
(c) forming a second electrode with a plurality of discreet layers of known thickness, each having etching properties different from at least one other;
(d) sequentially etching a calculated number of the discreet layers thereby incrementally reducing the resonator overall thickness by a known amount to adjust the resonator resonant frequency to the desired resonant frequency.
The distinct layers are composed of materials that have different etching properties and have thickness calculated to represent a selected fractional increment of the resonant frequency.
More particularly the present method includes first forming the second electrode with an initial conductive electrode layer having a thickness calculated to produce a resonator having a first resonant frequency that is higher than the desired resonant frequency. Subsequently, calculating a desired thickness for an adjustment layer such that when the adjustment layer is placed over the first conductive layer the resonant frequency of the resonator is reduced by a selected frequency increment. This selected frequency increment is a small fraction of the desired frequency correction for the resonator.
Having determined the thickness and number of adjustment layers to produce over the conductive layer sufficient to bring the top electrode thickness to a point such that the resonant frequency of the resonator is below the desired resonant frequency, each of these layers is created using materials having etching properties different from the etching properties of any adjacent adjustment layers. Then, the actual resonant frequency of the resonator is measured and the number of adjustment layers to be removed to incrementally adjust the actual resonator frequency to the desired resonant frequency determined.
Once this number is known, the process according to this invention comprises sequentially selectively etching the calculated number of adjustment layers to adjust the resonator resonant frequency to a desired frequency.
The terms xe2x80x9cdifferent etching propertiesxe2x80x9d and xe2x80x9cselective etchingxe2x80x9d as used herein mean that the materials used may be etched using an etching process for one that does not effect the other, so that one material can be removed completely without substantially effecting the other. Thus selective etching is the process of subjecting two or more materials to an etching process that effects only one of the materials.
It is a further objective of this invention to provide a method as hereinabove described, wherein there are at least two resonators electrically connected and wherein the step of forming said adjustment layers comprises forming a first plurality of stacked alternating adjustment layers having first and second etching properties on one of said at least two resonators, and forming a second plurality of stacked alternating adjustment layers having third and fourth etching properties, and alternatively selectively etching said first and said second pluralities of alternating stacked layers to remove said calculated number of adjustment layers to adjust the resonator resonant frequency to a different desired frequency for each of said at least two resonators.
It is also an object of the present invention to provide a mechanical resonator comprising a first electrode, a transducer and a second electrode wherein the second electrode comprises a conductive layer and a plurality of distinct stacked adjustment layers, each of the adjustment layers having distinct etching properties from any adjacent adjustment layers. Preferably, the first electrode is a bottom electrode placed over a supporting substrate, and the second electrode is a top electrode over the transducer.
The mechanical resonator resonant frequency is a function of the resonator thickness and the stacked adjustment layers each have a thickness such that removal of an adjustment layer increases the resonant frequency by a known increment.
The adjustment layer thickness may be uniform for all layers, or may decrease for adjustment layers closest to the conductive layer.